.\"	$OpenBSD: SSL_CTX_set_client_cert_cb.3,v 1.4 2018/03/27 17:35:50 schwarze Exp $
.\"	OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100
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.Dd $Mdocdate: March 27 2018 $
.Dt SSL_CTX_SET_CLIENT_CERT_CB 3
.Os
.Sh NAME
.Nm SSL_CTX_set_client_cert_cb ,
.Nm SSL_CTX_get_client_cert_cb
.Nd handle client certificate callback function
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft void
.Fo SSL_CTX_set_client_cert_cb
.Fa "SSL_CTX *ctx"
.Fa "int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey)"
.Fc
.Ft int
.Fo "(*SSL_CTX_get_client_cert_cb(SSL_CTX *ctx))"
.Fa "SSL *ssl" "X509 **x509" "EVP_PKEY **pkey"
.Fc
.Ft int
.Fn "(*client_cert_cb)" "SSL *ssl" "X509 **x509" "EVP_PKEY **pkey"
.Sh DESCRIPTION
.Fn SSL_CTX_set_client_cert_cb
sets the
.Fa client_cert_cb()
callback that is called when a client certificate is requested by a server and
no certificate was yet set for the SSL object.
.Pp
When
.Fa client_cert_cb
is
.Dv NULL ,
no callback function is used.
.Pp
.Fn SSL_CTX_get_client_cert_cb
returns a pointer to the currently set callback function.
.Pp
.Fn client_cert_cb
is the application-defined callback.
If it wants to set a certificate,
a certificate/private key combination must be set using the
.Fa x509
and
.Fa pkey
arguments and 1 must be returned.
The certificate will be installed into
.Fa ssl .
If no certificate should be set,
0 has to be returned and no certificate will be sent.
A negative return value will suspend the handshake and the handshake function
will return immediately.
.Xr SSL_get_error 3
will return
.Dv SSL_ERROR_WANT_X509_LOOKUP
to indicate that the handshake was suspended.
The next call to the handshake function will again lead to the call of
.Fa client_cert_cb() .
It is the job of the
.Fa client_cert_cb()
to store information
about the state of the last call, if required to continue.
.Pp
During a handshake (or renegotiation)
a server may request a certificate from the client.
A client certificate must only be sent when the server did send the request.
.Pp
When a certificate has been set using the
.Xr SSL_CTX_use_certificate 3
family of functions,
it will be sent to the server.
The TLS standard requires that only a certificate is sent if it matches the
list of acceptable CAs sent by the server.
This constraint is violated by the default behavior of the OpenSSL library.
Using the callback function it is possible to implement a proper selection
routine or to allow a user interaction to choose the certificate to be sent.
.Pp
If a callback function is defined and no certificate was yet defined for the
.Vt SSL
object, the callback function will be called.
If the callback function returns a certificate, the OpenSSL library
will try to load the private key and certificate data into the
.Vt SSL
object using the
.Fn SSL_use_certificate
and
.Fn SSL_use_private_key
functions.
Thus it will permanently install the certificate and key for this SSL object.
It will not be reset by calling
.Xr SSL_clear 3 .
If the callback returns no certificate, the OpenSSL library will not send a
certificate.
.Sh SEE ALSO
.Xr ssl 3 ,
.Xr SSL_clear 3 ,
.Xr SSL_CTX_add_extra_chain_cert 3 ,
.Xr SSL_CTX_use_certificate 3 ,
.Xr SSL_free 3 ,
.Xr SSL_get_client_CA_list 3
.Sh HISTORY
.Fn SSL_CTX_set_client_cert_cb
and
.Fn SSL_CTX_get_client_cert_cb
first appeared in SSLeay 0.6.6 and have been available since
.Ox 2.4 .
.Sh BUGS
The
.Fa client_cert_cb()
cannot return a complete certificate chain;
it can only return one client certificate.
If the chain only has a length of 2,
the root CA certificate may be omitted according to the TLS standard and
thus a standard conforming answer can be sent to the server.
For a longer chain, the client must send the complete chain
(with the option to leave out the root CA certificate).
This can be accomplished only by either adding the intermediate CA certificates
into the trusted certificate store for the
.Vt SSL_CTX
object (resulting in having to add CA certificates that otherwise maybe would
not be trusted), or by adding the chain certificates using the
.Xr SSL_CTX_add_extra_chain_cert 3
function, which is only available for the
.Vt SSL_CTX
object as a whole and that therefore probably can only apply for one client
certificate, making the concept of the callback function
(to allow the choice from several certificates) questionable.
.Pp
Once the
.Vt SSL
object has been used in conjunction with the callback function,
the certificate will be set for the
.Vt SSL
object and will not be cleared even when
.Xr SSL_clear 3
is called.
It is therefore
.Em mandatory
to destroy the
.Vt SSL
object using
.Xr SSL_free 3
and create a new one to return to the previous state.
